If it is saying "Finished Rendering" and the dots remain on the screen, then the only explanation I can think of is that you have it set to Write a GI Cache File. I know you say you've already checked that, but I don't know what else would cause it. Even if your camera were located somewhere that would render all-black, you should still at least see the dots disappear and be replaced by black before it says "Finished". Do you have multiple render nodes in your scene?